1. Field of the Invention
The present invention relates to a liquid crystal display driving system suited for use in a liquid crystal display device utilizing a ferroelectric liquid crystal.
2. Description of the Prior Art
FIG. 8 of the accompanying drawings schematically illustrates a liquid crystal display device 1 referred to both in the description of the prior art and the description of an embodiment of the present invention. The illustrated liquid crystal device 1 comprises a number m of scanning electrodes L1, L2, . . . , Lm (hereinafter, these scanning electrodes being collectively referred to by L) and a number n of signal electrodes S1, S2, . . . , Sn (hereinafter, these signal electrodes being collectively referred to by S). The sets of electrodes are laid so as to intersect with each other in the form of a matrix of columns and rows. A picture element Apj (p=l, 2, . . . , m, and j=l, 2, . . . , n) made of ferroelectric liquid crystal is disposed at each point of intersection of the scanning and signal electrodes L and S. The scanning electrodes L are applied with respective voltages, of arbitrary level, from a scanning electrode drive circuit 2. Further, the signal electrodes S are applied with respective voltages of arbitrary level from a signal electrode drive circuit 3.
The liquid crystal display device 1 utilizing the ferroelectric liquid crystal exhibits such a characteristic that, when a voltage exceeding a predetermined positive first defined voltage Va is applied to an arbitrary picture element Apj for a length of time greater than the unit time r (second), the picture element Apj is in a bright memory state. However, when a voltage not higher than a predetermined negative second defined voltage -Vb is applied to an arbitrary picture element Apj for a length of time greater than the unit time r (second), the picture element Apj is in a dark memory state.
FIGS. 9 and 10 are diagrams showing waveforms used to describe the principle of the liquid crystal driving system according to a typical prior device.
(1) and (2) shown in FIG. 9 illustrate selection voltages D1p and non-selection voltage H1k applied to an arbitrary scanning electrode Lp (p=l, 2, . . . m) and to the other scanning electrodes Lk (k.noteq.p) than the scanning electrode Lp, respectively, during a selection period Tap in which the arbitrary scanning electrode Lp is selected. The selection period Tap is set to be of a length four times the unit time, that is 4 r. The initial unit time r during this selection period Tap is hereinafter referred to as a first time span r1. Similarly, the subsequent second to fourth unit time r during the selection period Tap are hereinafter referred to as second to fourth time span r2 to r4, respectively.
In the selection voltage D1p shown by (1) in FIG. 9, a voltage V1 is set in the first and fourth time spans r1 and r4 of the selection period Tap and a voltage V8 is set in the second and third time spans r2 and r3 of the selection period Tap. On the other hand, in the non-selection voltage H1K shown by (2) in FIG. 9, a voltage V6 is set in the first and fourth time spans r1 and r4 of the selection period Tap and a voltage V3 is set in the second and third time spans r2 and r3 during the selection period Tap. It is to be noted that the voltages V1 and V8 in the selection voltage D1p and the voltages V3 and V6 in the non-selection voltage H1K have the following respective relationships. EQU V8=-V1 (1) EQU V6=-V3 (2)
(3) and (4) shown in FIG. 9 illustrate respective waveforms of a write voltage W1 and an erase voltage E1 applied to an arbitrary signal electrode Sj (j=1, 2, 3, . . . , n) during the selection period Tap in which the scanning electrode Lp is selected. The arbitrary electrode Sj is always applied with either the write voltage W1 or the erase voltage El. When the selection voltage D1p is applied to a scanning electrode, the write voltage W1 is applied to a scanning electrode, and the relevant picture element is set in a bright memory state, but in the event that the erase voltage El is applied, the relevant picture element is set in a dark memory state.
The write voltage W1 shown by (3) in FIG. 9 is set to a voltage V5, V4, V2 or V7 during the first time span r1, the second time span r2, the third time span r3 or the fourth time span r4, respectively, of the selection period Tap. On the other hand, the erase voltage E1 shown by (4) in FIG. 9 is set to a voltage V7, V2, V4 or V5 during the time span r1, r2, r3 or r4, respectively, of the selection period Tap. It is to be noted that the voltages V5, V4, V7 and V2 to which the write voltage W1 and the erase voltage E1 are set have the following relationships. EQU V5=-V4 (3) EQU V7=-V2 (4)
(1) shown in FIG. 10 illustrate a waveform of a write driving voltage Wpj applied to the picture element Apj when, during the selection period Tap, the selection voltage D1p and the write voltage W1 are applied to the scanning electrode Lp and the signal electrode Sj, respectively.
This write driving voltage Wpj is set by a difference between the selection voltage D1p and the write voltage W1. Further, it is of a level where the voltage level (V1-V7) of the fourth time span r4 exceeds the first defined voltage Va. Accordingly, the picture element Apj is in the bright memory state during this selection period Tap. It is to be noted that the voltage levels during the first time span r1 and the fourth time span r4 can be expressed as follows in consideration of the equations (1) to (4); EQU V8-V4=-(V1-V5) (5) EQU V8-V2=-(V1-V7) (6)
and, accordingly, a direct current component during the selection period Tap can be cancelled.
(2) shown in FIG. 10 illustrates a waveform of a leakage voltage Mkj applied to a picture element Akj in the event that, during the selection period Tap, the non-selection voltage H1K and the write voltage W1 are respectively applied to the scanning electrode Lk and the signal electrode Sj. The voltage level of leakage voltage Mkj during the first time span r1 to the fourth time span r4 can be expressed as follows consideration of the equations (1) to (4); EQU V6-V5=-(V3-V4) (7) EQU V3-V2=-(V6-V7) (8)
and, accordingly, a direct current component of the applied voltage Mkj during this selection period Tap can be cancelled.
(3) shown in FIG. 10 illustrates a waveform of an erase driving voltage Epj applied to the picture element Apj in the event that, during the selection period Tap, the selection voltage D1p and the erase voltage E1 are applied respectively to the scanning electrode Lp and the signal electrode S.sub.j. This erase driving voltage E.sub.pj is set so that the voltage level (V1-V5) in the fourth time span r4 does not exceed the first defined voltage V.sub.a. In the liquid display device 1 using normal ferroelectric liquid crystals, because the value of the second defined voltage -V.sub.b falls, in the absolute value, within the range of 0.8 to 1.2 times the first defined voltage V.sub.a, when the voltage (V8-V2) is applied in the second time span r2, and the voltage (V8-V4) is applied in the third time span r3, it is like a voltage of 1.2 times (V8-V2) being applied during the unit time span r. Thereby, the picture element is put in a dark memory state, (4) shown in FIG. 10 illustrates a waveform of a leakage voltage Nkj applied to the picture element Apj in the event that, during the selection period Tap, the non-selection voltage H1K and the erase voltage E1 are applied respectively to the scanning electrode Lk and the signal electrode Sj.
As is the case with the write driving voltage Wpj and the leakage voltage Mkj shown by (1) and (2) in FIG. 10, respectively, respective direct current components of the erase driving voltage Epj and the leakage voltage Nkj are cancelled.
FIG. 11 is a diagram showing waveforms of voltages applied to the liquid crystal display device 1 according to the typical prior art liquid crystal driving system. It is to be noted that, for the sake of brevity, the liquid crystal display device 1 is shown as having 4.times.4 picture elements Apj (p, j=1, 2, 3, 4).
(1) and (2) shown in FIG. 11 represent respective waveforms of voltages VL1 and VL2 applied to the scanning electrodes L1 and L2. Further, and (4) shown in FIG. 11 represent respective waveforms of voltages VS1 and VS2 applied to the signal electrodes S1 and S2. Dependent upon the voltages VL1 and S1 applied respectively to the scanning electrode L1 and the signal electrode S1, a voltage (VL1-VS1) of a waveform shown by (4) in FIG. 11 is applied to the picture element A11. Similarly, voltages (VL2-VS1) and (VL1-VS2) of waveforms shown by (5) and (7) in FIG. 11 are applied to the picture elements A21 and A12, respectively.
It is to be noted that, during the time period from the timing t0 to the timing t4, selection periods Ta1 to Ta4 are defined during which the scanning electrodes L1 to L4 are respectively selected. By way of example, during the selection period Ta1, the picture element A11 is set in the dark memory state and the picture element A12 is set in the bright memory state.
FIG. 12 is a diagram showing waveforms used to describe the principle of another prior art liquid crystal driving system.
(1) to (4) shown in FIG. 12 represent waveforms of a selection voltage D2p, a non-selection voltage H2k, a write driving voltage W2 and an erase driving voltage E2 which correspond to the waveforms (1) to (4) shown in FIG. 9, respectively. In this driving system, the selection period Tbp during which arbitrary scanning electrodes Lp (p=1, 2, 3, . . . , m) are selected is set to be twice the previously mentioned unit time r, that is, 2 r seconds.
FIG. 13 is a diagram showing respective waveforms of voltages applied according to the waveforms shown in FIG. 12 to the liquid crystal display device 1 of a construction including the 4.times.4 picture elements Apj (p, j=l, 2, 3, 4). (1) to (7) shown in FIG. 13 represent respective waveforms of voltages which correspond respectively to the waveforms (1) to (7) shown in FIG. 11. In this driving system, each selection period Tb1 to Tb4 shown from the timing t7 to the timing t11 is set to be twice the unit time. That is, 2 r, the write/erase operation of each of the picture elements is reduced to half that required in the previously mentioned first driving system.
In the event that the same picture is continuously displayed by the former driving system, and if the liquid crystal display device 1 utilizing the ferroelectric liquid crystal is of a construction employing the 4.times.4 picture elements, such a voltage as shown by (4) in FIG. 11 is applied to the picture element which maintains a dark display. The relationship between this applied voltage and the brightness of the picture element is shown by (1) and (2) in FIG. 14. Since the voltage applied to the picture element A11, during the period Ta1 in which the selection voltage D1p is applied to the scanning electrode L1, once exceeds the voltage Va with which the picture element is set in the bright memory state, and then causes the picture element to be in the dark memory state, the brightness of such picture element exhibits a peak A.
A time span TF1 from the occurrence of this peak A to the next succeeding occurrence of a peak A coincides with the time span from the selection of the scanning electrode L1 to the next succeeding selection of the same scanning electrode L1. Using the time 4 r (s) during which the scanning electrode Lp is selected and the number m of the scanning electrodes, the following relationship can be established. EQU TF1=4r.times.m (9)
Since human eyes are sensitive to light of a frequency higher than 1/60 second, the following condition has to be satisfied in order for the light not to be perceived. EQU TF1=4r.times.m.ltoreq.1/60 (10)
While in the example of FIG. 4 there will be no problem since the number m is 4, the unit timer (s) required to change the memory state when the number m is 200 will be as expressed below: EQU r.ltoreq.1/60.times.1/4m.apprxeq.20.8 (.mu.s) (11).
This is a value difficult for the existing ferroelectric liquid crystal to achieve. The reality is that, since the unit time r is about equal to 100 .mu.s, the number m of the scanning electrodes that can be displayed is about 41, to wit: EQU m.ltoreq.1/60.times.1/4r.apprxeq.41.7 (12).
Also, such a voltage as shown by (7) in FIG. 11 is applied to the picture element which continues a bright display. The relationship between this applied voltage and the brightness of the picture element is such as shown by (3) and (4) in FIG. 14, similarly exhibiting a peak B. Therefore, TF1 must be smaller than 1/60 (s).
In the event that the same picture is continuously displayed by the latter driving system, and if the liquid crystal display device 1 utilizing the ferroelectric liquid crystal is of a construction employing the 4.times.4 picture elements, such a voltage as shown by (4) in FIG. 13 is applied to the picture element which continues a dark display. The relationship between this applied voltage and the brightness of the picture element is shown by (1) and (2) in FIG. 15. In this case, although the picture element need not be set in the bright memory state, a peak C occurs in the brightness thereof. In such case, the time span TF2 from the occurrence of this peak C to the next succeeding occurrence of a peak C, the time period 2 r (s) during which the scanning electrode Lp is selected, and the number m of the scanning electrodes give the following relationship: EQU TF2=2 r.times. (13).
Accordingly, when the number m is 200, the unit time r gives the following relationship: EQU r.ltoreq.1/60.times.1/2m 41.3 (.mu.s) (14).
Even this is a value difficult for the existing ferroelectric liquid crystal to achieve. Conversely, when the unit time r is chosen to be 100 .mu.m, the number m of the scanning electrodes will be about 83, to wit: EQU m.ltoreq.1/60.times.1/2r.ltoreq.83.3 (15).
Also, such a voltage shown by (7) in FIG. 13 is applied to the picture element which continues a bright display. The relationship between this applied voltage and the brightness of the picture element gives such as shown by (3) and (4) in FIG. 15. This results in the occurrence of a peak D in the brightness, requiring TF2 to be smaller than 1/60 (s).